Commercially available systems that recover energy from waste products frequently use a rotary kiln in which waste fuel is combusted under excess air conditions. In these rotary kiln systems, more than twice the stoichiometric amount of air is frequently supplied to the rotary kiln to ensure complete combustion of the waste fuel. The excess-air rotary kiln systems can completely combust virtually any kind of fuel because the fuel is turned over repeatedly. Rotary kiln systems can effectively burn carbohydrates, hydrocarbons and even hard to burn fixed carbon. A typical rotary kiln might have a 12 foot diameter, a 55 foot length.
In excess-air rotary kiln systems, almost all of the combustion takes place in the rotary kiln, but a secondary combustion chamber is needed to complete combustion of unburned products in the flue gas, such as carbon monoxide, unburned hydrocarbons or particulate matter. The temperature of combustion gases in the secondary combustion chamber is typically between 1,800.degree.-2,200.degree. F. A supplemental fuel such as natural gas or fuel oil is used to ensure complete combustion in the secondary combustion chamber. Hot combustion gases from the secondary combustion chamber are normally ducted to a heat recovery boiler where useful energy is recovered in the form of steam. The flue gas is then discharged through an air pollution control device (APCD) and a stack to the atmosphere. An induced draft fan draws air and flue gases through the entire system.
Waste fuel can be provided to the rotary kiln in a batch manner using a ram feeder, in a continuous manner using augers, or in many other ways known in the art. The rotary kiln is typically ignited with a start up burner that can sometimes be used as a supplemental burner to burn supplemental fuels, such as natural gas, fuel oil, propane, or the like.
The rotary kiln is slightly inclined so that the fuel feed end in which the waste fuel is introduced is slightly higher than the ash discharge end from which combustion ash is discharged. One advantage of rotary kilns is that lime can be injected into the kiln to provide for in-bed scrubbing for SO.sub.x. In the prior art, combustion air is introduced into the rotary kiln through the fuel feed end, and the hot combustion gases are ducted from the ash discharge end of the rotary kiln to the secondary combustion chamber. The fuel and combustion air, thus, flow in a co-current configuration.
The induced fan is set to draw excess air into the rotary kiln (i.e., about twice the stoichiometric amount for combustion in the kiln). A substantial amount of flue gas is thus discharged to the atmosphere, and most of the flue gas is nitrogen. The temperature of the flue gas exiting the stack is typically at least 400.degree. F., and represents a significant loss of energy to the atmosphere. A conventional excess-air rotary kiln system can recover in the form of steam about 60-65% of the heating value in the waste fuel.